1. Field of the Invention
The present invention relates generally to a space-time coding apparatus and method in a mobile communication system, and in particular, to a space-time coding apparatus and method using a non-binary coding technique.
2. Description of the Related Art
In communication systems, it is very important to efficiently and reliably transmit data over a channel. In a next generation mobile communication system being recently developed, it is necessary to increase system efficiency by using an appropriate channel coding technique in order to support high-speed communication services capable of processing and transmitting various information such as image and radio data, which has evolved from an early voice-oriented service.
However, unlike a wired channel environment, a wireless channel environment existing in a mobile communication system suffers from information loss due to inevitable errors occurring because of multipath interference, shadowing, wave attenuation, time-varying noise, interference, fading, and the like. The information loss causes serious distortion after actual signal transmission, resulting in deterioration in the overall system performance. Generally, in order to reduce the information loss, various error control techniques are used according to channel characteristics to increase system reliability, and among the error control techniques, the most typical technique is a technique using an error correction code. The error correction code includes a Reed-Solomon code, a convolutional code, and a turbo code.
In order to resolve communication instability problems due to fading, diversity techniques are used, and space diversity technology, which is a typical diversity technique uses multiple antennas. The space diversity technology is classified into a reception antenna diversity technique employing multiple reception antennas, a transmission antenna diversity technique employing multiple transmission antennas, and a multiple-input multiple-output (MIMO) technique employing multiple reception antennas and multiple transmission antennas. A space-time coding (STC) technology is a type of MIMO technique, and in the STC technology, a signal encoded in a predetermined coding technique is transmitted via multiple transmission antennas to extend the coding technology from a time domain to a space domain, thereby achieving a low error rate. With reference to FIG. 1, a description will now be made of a structure of a transceiver using the STC technology (hereinafter referred to as an “STC transceiver”) for a mobile communication system.
FIG. 1 is a diagram schematically illustrating a structure of a general STC transceiver. As illustrated in FIG. 1, the STC transmitter is comprised of an STC encoder 100 and a plurality of transmission antennas 110 to 114 for transmitting signals output from the STC encoder 100, and the STC receiver includes a plurality of reception antennas 120 to 124 each receiving signals transmitted from the transmission antennas 110 to 114, and an STC decoder 102.
The STC encoder 100 encodes input information data according to a given code rate. If the number of bits of the input information data is K and the number of bits constituting a symbol output from the STC encoder 100 is N, the code rate is K/N. That is, the rate=K/N and STC encoder 100 receives K-bit information data and outputs an N-bit symbol. Symbols output from the STC encoder 100 are sequentially transmitted via the multiple transmission antennas 110 to 114.
The multiple reception antennas 120 to 124 each receive symbols transmitted from the multiple transmission antennas 110 to 114. That is, the reception antenna 120 receives symbols transmitted via the transmission antennas 110 to 114, the reception antenna 122 receives symbols transmitted via the transmission antennas 110 to 114, and in this manner, the last reception antenna 124 receives symbols transmitted via the transmission antennas 110 to 114.
The STC decoder 102 decodes the symbols received via each of the multiple reception antennas 120 to 124 according to a predetermined decode rate. The decode rate of the STC decoder 102 is determined according to the code rate of the STC encoder 100. That is, if the code rate of the STC encoder 100 is K/N, the decode rate of the STC decoder 102 is given as N/K. The STC decoder 102 decodes the received symbols to output the signals transmitted from the transmission antennas 110 to 114. Therefore, a system with a low error rate can be implemented depending on how to design the STC encoder 100, and in this way, it is possible to increase system reliability.
FIG. 2 is a diagram schematically illustrating a structure of a general STC transmitter using a turbo code. Herein, the STC transmitter using a turbo code will be referred to as a “turbo STC transmitter,” and it is assumed that a code rate of the turbo STC transmitter is ⅓. The turbo STC transmitter is comprised of a first constituent encoder 200, an interleaver 202, a second constituent encoder 204, and a plurality of, for example, 3 transmission antennas 206 to 210.
When information data is received, the received information data is forwarded to the first constituent encoder 200 and the interleaver 202. The turbo interleaver 202 interleaves the received information data according to a predetermined interleaving rule and outputs the interleaved information data to the second constituent encoder 204. The first constituent encoder 200 encodes the received information data according to a predetermined coding technique, and the second constituent encoder 204 encodes the interleaved information data according to a predetermined encoding technique. The received information data, or a systematic symbol S, is transmitted, as it is, to a reception side via the transmission antenna 206. An output signal, or a first parity symbol P1, of the first constituent encoder 200 is transmitted to the reception side via the transmission antenna 208. An output signal, or a second parity symbol P2, of the second constituent encoder 204 is transmitted to the reception side via the transmission antenna 210. In a conventional non-STC mobile communication system, the systematic symbol S, the first parity symbol P1 and the second parity symbol P2 are added up by a separate adder and then transmitted to the reception side via one transmission antenna. However, in the STC transmitter using multiple transmission antennas, the systematic symbol S, the first parity symbol P1 and the second parity symbol P2 are separately transmitted via different transmission antennas.
FIG. 3 is a diagram schematically illustrating a structure of a turbo STC receiver matched to the turbo STC transmitter of FIG. 2. Signals, or symbol streams, transmitted via multiple transmission antennas of the turbo STC transmitter are received at the turbo STC receiver via its reception antenna. Here, the turbo STC receiver can have either one reception antenna or multiple reception antennas. For example, if the number of reception antennas of the turbo STC receiver is 3, each of the 3 reception antennas receives symbol streams transmitted from the 3 transmission antennas of the turbo STC transmitter.
As illustrated in FIG. 3, the turbo STC receiver is comprised of two constituent decoders, a first constituent decoder 300 and a second constituent decoder 306, two deinterleavers 302 and 304, and an interleaver 308. A signal transmitted from the turbo STC transmitter is forwarded to the first constituent decoder 300 and the deinterleaver 304. The first constituent decoder 300 performs a decoding operation only on the first parity symbol P1, or a symbol transmitted via the transmission antenna 208. A signal output from the first constituent decoder 300 is input to the deinterleaver 302. The deinterleaver 302 receives the signal output from the first constituent decoder 300, deinterleaves the received signal according to the interleaving rule employed in the interleaver 202, and outputs the deinterleaved signal to the second constituent decoder 306.
The deinterleaver 304 receives the second parity symbol P2, or a signal transmitted via the transmission antenna 210. The deinterleaver 304 deinterleaves the received signal according to the interleaving rule employed in the interleaver 202, and outputs the deinterleaved signal to the second constituent decoder 306. The second constituent decoder 306 decodes the signals received from the deinterleaver 304 and the deinterleaver 302, and outputs the decoded signals to the interleaver 308. The interleaver 308 interleaves the signal output from the second constituent decoder 306 according to the interleaving rule employed in the interleaver 202, and outputs the interleaved signal to the first constituent decoder 300. By repeatedly performing the decoding process, i.e., through an iterative decoding operation, the turbo STC receiver can correctly decode a signal transmitted from the transmitter.
As described above, in the STC technology in which the transmission side uses a plurality of transmission antennas, if a signal transmitted via a particular one of the transmission antennas suffers from distortion, data transmitted by the transmission side is decoded using signals transmitted from other transmission antennas with the exception of the particular transmission antenna. For example, in a system that transmits a signal using 3 transmission antennas, if a signal transmitted from a particular one of the 3 transmission antennas suffers from distortion, the reception side decodes the signal transmitted from the transmission side using signals transmitted from the other 2 transmission antennas which have not suffered from distortion. In this case, the decoding efficiency is lower than when the signal transmitted from the transmission side is decoded using signals transmitted from the 3 transmission antennas.
Meanwhile, a hybrid automatic retransmission request (HARQ) scheme is used to perform retransmission when an error has occurred in a received signal. The HARQ scheme requests retransmission until no error occurs in the received signal, to thereby implement a high-reliability communication system. The HARQ scheme employs a soft combining technique in order to increase efficiency, and in the soft combining technique, the reception side temporarily stores defective data in a soft buffer and later, combines the stored defective data with corresponding retransmitted data, thereby reducing an error rate. The soft combining technique is classified into a chase combining (CC) technique and an incremental redundancy (IR) technique. In the CC technique, the transmission side uses the same format for both initial transmission and retransmission, and in the IR technique, the transmission side uses different formats for initial transmission and retransmission. In the IR technique, when n-bit user data is channel-coded into m symbols, the transmission side transmits only some of the m symbols at initial transmission, and then sequentially transmits the remaining symbols at retransmission. That is, the initial transmission is different from the retransmission in coding rate. In response, the reception side attaches the retransmitted symbols to the rear of the initially-transmitted symbols to construct high-coding rate symbols, and then performs error correction on the combined symbols.
As described above, in the IR technique, the turbo STC transmitter transmits only some of transmission symbols by employing a puncturing technique. For example, if the HARQ scheme is applied to the turbo STC transmitter, the signal output from the first constituent encoder 200 is not transmitted for an even time period and the signal output from the second constituent encoder 204 is not transmitted for an odd time period. That is, for the received information data, the turbo STC transmitter transmits one parity symbol rather than two parity symbols. The IR-based HARQ scheme is not suitable for a system that transmits a signal using three or more transmission antennas, because two symbols, or bit streams, are transmitted at a particular time slot in the IR-based HARQ scheme. In addition, because the signals transmitted via the multiple transmission antennas as shown in FIG. 2 are transmitted via independent paths, there is no cross correlation between the signals transmitted via the multiple transmission antennas. Therefore, if an error occurs in a signal transmitted via a particular one of the multiple transmission antennas, the reception side has low decoding performance for the signal transmitted from the transmission side.